Mysterious, nearly invisible objects known as “dark comets‘ may pose a greater threat to Earth than scientists thought, new research suggests.
These small, rapidly rotating objects wander near Earth, possibly after migrating from more distant parts of the Solar System. They can be a source of water and other volatile elements, and also a powerful source of danger.
Usually comets are very different from asteroids. Comets come from the outermost region of the Solar System, where temperatures are low enough to allow molecules such as water to freeze. While comets usually have stable orbits, they can sometimes be disrupted by gravitational interactions with the giant planets, sending some of the icy rocks spiraling inward solar system. When they do, heat from the sun causes them to disintegrate, a process that also gives comets signature tails.
Asteroids, on the other hand, usually live in the inner solar system, usually between Mars and Jupiter. They are much more rocky than their cometary cousins and therefore can survive much longer under the sun’s glare. But they also occasionally fall into unstable orbits that lead them dangerously close to Earth.
But there is a strange, third kind of space rock that astronomers have only recently begun to identify: dark comets that behave both like asteroids and comets. Now, in a paper accepted for publication in the journal Icarus, a team of astronomers has attempted to identify the mysterious origins of dark comets.
Dark comets are small – only tens of kilometers in diameter. They show no visible evolution of gases or evaporation of volatile elements such as water. But they don’t move in perfect orbits either. Instead, they show evidence of “non-gravitational” acceleration, suggesting that there are some other forces capable of nudging their orbits slightly.
All small objects in the Solar System, including asteroids, have some non-gravitational acceleration, but astronomers can usually identify the cause. For example, asteroids are heated unevenly by the sun, which causes a small but measurable change in their orbits.
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The researchers found that the non-gravitational acceleration of dark comets is not compatible with non-uniform heating, so there must be another source of acceleration. The team believes that dark comets do emit gas that can cause their own non-gravitational acceleration, just at an undetectable level.
Dark comets also spin very quickly, which means they must have enough internal strength to keep from tearing apart. From this, the researchers concluded that dark comets have a similar composition to asteroids and are likely the result of the fragmentation of a larger object.
Based on these lines of evidence, researchers suspect that dark comets likely originate in the main asteroid belt between Mars and Jupiter and are disrupted from their orbits by gravitational interactions with Saturn. So dark comets are probably asteroids, but with a strange flavor—they’re asteroids loaded with an unusually large amount of light molecules, like water, that can evaporate as the objects enter the inner solar system. The researchers therefore suggest that dark comets may be a possible candidate for how early Earth got its water.
Meanwhile, the unstable orbits of dark comets and their unusual combination of properties make them particularly dangerous near-Earth objects. They are small, fast and hard to detect. Most importantly, they don’t behave like their more familiar cousins asteroids and comets, making them unpredictable, the researchers found.
To help protecting the Earth from possible threatswe will need to study rogue populations like dark comets in much more detail to better detect them and predict their future motions.
